U.S. Pat. No. 5,506,180 describes thermally stable and chemically resistant glasses, amongst other things for use as TFT display glass. On account of the high B.sub.2 O.sub.3 contents of 10% or more and the relatively low SiO.sub.2 levels (46-56%), glasses of this type are not sufficiently resistant to hydrochloric acid. Furthermore, their resistance to solutions containing hydrofluoric acid is only moderate. The matching of the thermal expansion to polycrystalline silicon is insufficient. The processing temperatures V.sub.A of less than 1150.degree. C. are too low to make it possible to use drawing methods such as the microsheet down-draw method and the overflow fusion method as alternatives to the float process. Furthermore, the specified strain points of 642.degree. C. or less are too low to ensure that there is little compaction.
European Patent EP 510 544 B1 describes alkali metal-free glass which can be made in the float process and, amongst other things, are used as a substrate for a variety of displays and photographic masks. The glasses are free of BaO and MgO and have only low levels of B.sub.2 O.sub.3. At 4.5-6.0 .times.10.sup.-6 /K, their thermal expansion is no longer sufficient to meet the requirements of high-quality TFT display glasses which are very well-tailored to polycrystalline silicon.
Patent EP 527 320 B1 describes flat display devices having a strontium aluminosilicate glass substrate. The glass compositions for glasses with high devitrification stability appear to be geared especially to the suitability for being made in the overflow fusion draw method. The density of the glasses and their coefficient of thermal expansion are too high.
Japanese JP 8-295530 A describes alkali metal-free glass substrates whose stability with respect to hydrochloric acid is lowered because of the high B.sub.2 O.sub.3 content, (up to 15% by weight).
PCT Applications WO 97/11919 and WO 97/11920 also describe alkali metal-free glass substrates. They are free of or low in MgO. They may respectively contain up to 5% by weight of ZnO and TiO.sub.2. ZnO can cause glass defects owing to its inclination to evaporate from the glass surface in the float bath and subsequently condense. The possibly high TiO.sub.2 content will cause a brown tint in the glasses when conventional raw materials are used, since the Fe.sup.3+ always present in the usual raw materials forms a brown color complex with Ti.sup.4+.
European Patent Application EP 714 862 A1 describes alkali metal-free glasses for use in TFT flat displays. The illustrative embodiments exhibit very good display characteristics, for example in terms of thermal expansion, strain point or HCl stability. However, the viscous behavior of the glasses is unfavorable for the allegedly suitable production process of "floating" as evinced by the fixed-point temperatures T (log .eta.=4)=1310-1350.degree. C. and T (log .eta.=2)=1710-1810.degree. C. which have a decisive technical effect on the floating as well as melting and refining methods, and are too high. For highly viscous aluminosilicate glasses of this type with high SiO.sub.2 contents, efficient refining is extremely problematic. The glasses do not therefore meet the requisite high demands in terms of visual quality.
EP 672 629 A2 or U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat displays. They present a variety of composition ranges with different coefficients of thermal expansion. These glasses are allegedly processable not only with the overflow fusion draw method but also with other flat glass production methods. However, in particular the glasses which have a coefficient of thermal expansion tailored to polycrystalline Si have very high processing temperatures V.sub.A, which make them unsuitable for the float process. As in the case of the glasses described above, the visual quality is here again not high, since no way of effective refining, in particular one compatible with the float process, is presented. The refining agents Sb.sub.2 O.sub.3 and As.sub.2 O.sub.3 mentioned by way of example are unsuitable for the float process because they can be reduced readily. The same is true for the optional glass components Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.5.
In the alkali metal-free glass substrates for TFT AMLCD from JP 9-48632 A as well, the visual quality will not be high, since merely SiO.sub.2, B.sub.2 O.sub.3, MgO and BaO are to be present in the glass.
German Patent DE 38 08 573 C2 describes alkali metal-free aluminosilicate glasses which contain SnO.sub.2, are easy to melt and can be refined economically. The glasses exhibit high chemical stability. They are used as photographic mask glass. At 4.0.times.10.sup.-6 /K, their thermal expansion behavior is not optimally tailored to that of polycrystalline silicon. On account of the fact that they are free of B.sub.2 O.sub.3 and at the same time have high Al.sub.2 O.sub.3 content, the glasses have a temperature/viscosity profile which is unfavorable for flat glass production processes.
German Patent DE 196 17 344 C1 in the name of the applicant company also describes alkali metal-free glasses which contain SnO.sub.2. The glasses have a relatively low SiO.sub.2 level and a high Al.sub.2 O.sub.3 content. With a coefficient of thermal expansion of about 3.7.times.10.sup.-6 /K and very good chemical stability, these glasses are suitable for use in display technology. There is, however, still a need for improvement in terms of making them economically with the float as well as the draw method, that is to say in terms of being "universally" manufacturable. This is also true as regards glasses from German Patent DE 196 036 98 C1 in the name of the applicant company.